The present invention relates to a novel oxidative scrubber system for removing mercury from plated solid substrates and more specifically to an oxidative scrubber system comprising an aqueous solution of an alkali metal chromate or alkali metal dichromate salt in combination with nitric acid.
The present invention also relates to a process for removing mercury from solid substrates such as in an empty cartridge casing and live ammunition in a safe and economical manner by utilizing in the process the novel oxidative scrubber system.
During the manufacture of small cartridge cases, the brass cartridge cases are checked for stress concentrations and hairline cracks, which if not detected, would result in the production of unacceptable ammunition. The procedure for checking the cartridge cases, which is set forth in Federal Test Method No. 151A and Method 831, involves cleaning the cases in dilute nitric acid, followed by immersion in one percent mercurous nitrate solution. Metallic mercury plates out on the surface of the casing, preferentially in cracks or highly stressed regions. The cases are then visually inspected or heated to remove the mercury from the bulk surface, followed by visual inspection of the pyrolyzed cases. The mercury cracking test is normally part of the quality control imposed after key production steps and as a part of the final acceptance of ammunition lots.
The casings and the live ammunition failing the mercury crack test are then sent to a furnace or a Tooele Deactivation Furnace where they are melted down and redrawn. During this operation mercury vapors are emitted in the atmosphere at unacceptable levels. In an effort to avoid this emission problem, the mercury must be removed from the cartridges and ammunition before it is sent to the furnace.
While there are several systems capable of removing mercury from brass cartridge cases, they all have serious deficiencies both technically and economically. For example, systems incorporating an oxidizer with various types of acids such as hydrochloric, sulfuric and perchloric were ineffective. Also ammonium persulfate and potassium iodide chemical removal systems displayed vast inefficiencies and could not be acceptably used in a commercial operation.
Other designs were also considered, such as utilizing a vacuum oven, which pyrolyzes the mercury from the cartridge cases, in series with a mechanically refrigerated mercury trap and adsorption column. However, in order to meet the effluent guidelines, it is still found necessary to remove the mercury from the cartridge before treatment in the oven.